The present invention relates to a ferrite magnet having high saturation magnetization as and coercive force iHc extremely useful for wide varieties of magnet applications, and a method for producing same.
Ferrite magnets are used for various applications including rotors of motors, generators, etc., and particularly ferrite magnets with higher magnetic properties are recently demanded for the purpose of miniaturization and reduction in weight in the field of rotors for automobiles and for the purpose of increasing efficiency in the field of rotors for electric appliance.
High-performance sintered magnets such as Sr ferrite or Ba ferrite have conventionally been produced by the following methods. First, iron oxide is mixed with carbonate of Sr or Ba and calcined to complete a ferrite-forming reaction. After clinker formed by calcination is coarse-pulverized, SiO.sub.2, SrCO.sub.3 or CaCO.sub.3 is added to control sintering behavior, and Al.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 is added to control coercive force iHc. Coarse particles with additives are then subjected to fine pulverization to an average particle size of 0.7-1.0 .mu.m. The resultant fine powder slurry is wet-molded in a magnetic field while orienting ferrite powder, to form a green body. The green body is then sintered and worked to final product shape.
In such a conventional production method, there are the following five methods conceivable to increase the performance of ferrite magnets.
The first method is to finely pulverize the ferrite magnet. Since it shows higher coercive force iHc as the size of the crystal grains in the sintered body becomes nearer about 0.9 .mu.m, which is a critical single magnetic domain size of the magnetoplumbite-type Sr ferrite magnet, fine pulverization is carried out to an average particle size of 0.7 .mu.m or less, for instance, taking into consideration grain growth during the sintering process. This method, however, is disadvantageous in that productivity decreases as the fine pulverization proceeds because the fine pulverization lowers water-removal characteristics in the wet molding.
The second method is to make the crystal grain size of the sintered body as uniform as possible. Ideally, the crystal grain size is made as uniformly close to the above critical single magnetic domain size (about 0.9 .mu.m) as possible. Crystal grains larger than this size contribute to decrease in the coercive force iHc. The specific means for achieving high performance in this method is to improve a particle size distribution of fine powder, but the improvement is naturally limited because known pulverizers such as ball mills, attritors, etc. are used in a commercial production. Recently, attempts have been made to produce fine ferrite powder having a more uniform particle size by a chemical precipitation method, but such methods are not suitable for commercial mass-production.
The third method is to increase the orientation of the crystal grains which affects magnetic anisotropy. The specific means in this method is to add a surfactant to a slurry of fine ferrite powder to improve the dispersion of ferrite particles in the slurry, or to increase the intensity of the magnetic field in the process of orientation.
The fourth method is to increase the density of the sintered body. Though the Sr ferrite has a theoretical density of 5.15 g/cm.sup.3, Sr ferrite magnets now commercially available have a density of about 4.9-5.0 g/cm.sup.3, 95-97% of the theoretical density. Improvement in the residual magnetic flux density Br is expected by increasing the density of the ferrite sintered body, special density-increasing methods such as HIP, etc. are needed to achieve as high density as exceeding the above range. Such special processes, however, lead to high production cost, depriving the ferrite magnets of advantages as inexpensive magnets.
The fifth method is to improve the saturation magnetization .sigma.s or crystal magnetization anisotropy constant of the ferrite compound per se which is a main component of the ferrite magnet. Improvement in the saturation magnetization .sigma.s is likely to directly lead to improvement in the residual magnetic flux density Br. Also, improvement in the crystal magnetization anisotropy constant is likely to directly lead to improvement in the coercive force iHc. Ferrite compounds which have conventionally been produced have a crystal structure of magnetoplumbite (M type). Though investigation has been conducted on W-type ferrite having larger saturation magnetization than that of the M-type ferrite, the W-type ferrite has not been mass-produced because it is difficult to control the sintering atmosphere.
Under such circumstances, higher performance of the ferrite magnets is now sought by the first to fourth methods. However, it is difficult to seek extremely higher performance in ferrite magnets mainly composed of compounds expressed by SrO.multidot.nFe.sub.2 O.sub.3, by the first to fourth methods. The first reason therefor is that the first to fourth methods include conditions which may hinder high productivity or steps which are difficult to be put into a mass-production. The second reason therefor is that the SrO.multidot.nFe.sub.2 O.sub.3 ferrite magnet has magnetic properties, particularly residual magnetic flux density Br, which are already close to the theoretical levels.